Modern wireless communications devices include a number of flip-chip assembled radio frequency (RF) power amplifiers, for example, typically implemented in packaged semiconductor devices or modules. The packaged semiconductor devices include multiple transistors, arranged on a semiconductor substrate, including a signal path of the RF power amplifiers. The transistors may be bipolar junction transistors (BJTs) or heterojunction bipolar transistors (HBTs), each of which includes a base, an emitter and a collector.
To handle the enormous dissipated power density in the transistors, the emitters of the transistors may be directly connected to a module printed circuit board (PCB) through metal (e.g., copper) pillars. The module PCBs may then be connected to a mother board, for example. For best thermal and electrical connection, the pillars are placed directly over the transistors, and more particularly, over the emitter and base portions of the transistors (which are typically stacked). In this configuration, the emitters are directly connected to the PCB through a vertical stack of metal comprising an on-die interconnect, the copper pillar and solder. This configuration provides substantially uniform thermal conductivity and compact, low-inductance electrical connections. However, the direct vertical metal stack also transmits significant strain to semiconductor junctions of the transistors (e.g., base-emitter junctions) due to plastic deformation of the metal stack, mold compound covering the transistors and the PCB during assembly of the RF front-end-module to the mother board. The semiconductor junctions are the junctions where materials forming the various components of a transistor meet. For example, in an NPN bipolar transistor, the base-emitter junction is the transition plane from the N-type-doped emitter to the P-type-doped base. In a HBT, the different layers meeting at the junction might be formed of different semiconductor materials, as well as different doping polarities. For example, a common RF power amplifier HBT may include an emitter with a wider bandgap (e.g., Indium gallium phosphide (InGaP)) in contact with a relatively narrower bandgap material (e.g., gallium arsenide (GaAs)) in the base. For example, base-emitter junctions may be in the regions that are below emitter strips, where the emitter strips meet a base mesa. The strain on the semiconductor junctions from such chip-package interaction alters the semiconductor bandgap, which alters the semiconductor junction turn-on voltage, which alters the RF performance at a fixed bias voltage, and subsequent relaxation of such strain can lead to changes in electrical performance across the product lifetime.
For example, strain may occur through chip-package interaction in assembly of the semiconductor module. During solder reflow, for example, to attach the RF front-end-module to the mother board, the module assembly has already been completed. Therefore, organic mold compound (e.g., with inorganic filler particles) has already been injected into spaces between a flipped power amplifier die and the module substrate. During attachment of the module to the mother board, the reflow cycle heats all the materials, causing them to expand, and the pillar solder joints melt, allowing the solder to elongate. In the reflow cool-down, the solder refreezes in an elongated state, while the mold compound (and other dielectrics) continue to contract, putting the pillars (and the semiconductor junctions directly connected to them) into compressive strain.
Accordingly, there is a need for providing metal pillars in the module PCBs that provide structural support and electrical conductivity, as well as heat dissipation, without causing undue strain on the semiconductor junctions.